WASHINGTON (AP) - Congressional investigators examining how preparations for the Sept. 11 attacks went undetected by the nation's law enforcement and intelligence agencies have been receiving less than full cooperation from those agencies, the chairman of the Senate Intelligence Committee says.

Sen. Bob Graham (news, bio, voting record), D-Fla., told reporters Wednesday that some requested documents weren't being turned over, and interviews of potential witnesses were taking place in intimidating environments. He declined to go into specifics.

Graham's comments pointed to just the latest troubles in Congress' investigation, being conducted jointly by the Senate and House intelligence committees.

Its director, former CIA (news - web sites) Inspector General L. Britt Snider, resigned after little more than two months on the job, apparently forced out over a personnel dispute with the Senate Intelligence Committee's vice chairman, Sen. Richard Shelby (news, bio, voting record), R-Ala.

Graham acknowledged Snider's departure had caused some interruption in the investigation.

The tumult has forced back the beginning of public hearings on the issue, once promised to take place as early as April. They now won't begin before June.

After hearing of Graham's comments, CIA spokesman Bill Harlow described the agency's cooperation with the investigation as "extensive, extraordinary and unprecedented."

"We've given them access to thousands of highly classified documents," he said. "We've given them briefings. We've given them information we have assembled, which, without our efforts, they would be unable to find. We've housed members of their staff in our headquarters. We've done all these things while we're fighting a war."

Justice Department (news - web sites) spokesman Mark Carallo said, "The department has been cooperating to the fullest extent possible with the congressional committees, and we will continue to do so. We are always willing to discuss concerns of members of Congress in hopes of resolving any perceived problems."

Graham acknowledged Justice officials told the committee that disclosing some documents could interfere with criminal investigations, but Graham said the committee regularly deals with classified materials and there's no chance those documents will be made public.

Graham said committee officials intend to take their complaints personally to Attorney General John Ashcroft (news - web sites) and CIA Director George Tenet.

"We thought we had from those highest levels the kind of assurances we would get cooperation," Graham said.

One official familiar with the investigation said some committee requests for information have required the compilation of hundreds of thousands of documents. But the investigation has uncovered no single missed piece of intelligence that would have allowed U.S. authorities to stop the attacks.

Graham said the committee may exercise its subpoena power to force cooperation.

The Democratic senator suggested the reluctance to cooperate reflected a tendency in the Bush administration, noting the administration's refusal to let Homeland Security Director Tom Ridge testify before Congress and its resistance to turning over documents relating to the administration's consultations on national energy policy.

Also Wednesday, Graham's committee approved an intelligence budget for 2003. Graham and committee officials declined to provide details. Shelby said it contained a "meaningful" increase in funding for the nation's intelligence community.

The total intelligence budget is kept secret, although open-government advocates managed to get the government to disclose its total in 1998: $26.7 billion. Since, it has been estimated to be around $30 billion.

Last year, the intelligence budget went up by about 8 percent. Officials said revitalizing the CIA's spy networks and improving the computers that analyze signals intelligence remain key goals in this year's budget.

This year, President Bush (news - web sites) was believed to have proposed increasing the CIA's budget, which only makes up a portion of the total intelligence budget, from about $3.5 billion to between $5 billion and $5.5 billion for 2003.

Much of that would pay for expanding CIA's corps of overseas case officers, hiring allies and equipping counterterrorism teams in foreign countries averse to having U.S. military advisers on their soil.

Other intelligence agencies included in the budget are the National Security Agency, which conducts electronic wiretapping and signals gathering for foreign intelligence purposes; the National Reconnaissance Office, which designs and operates spy satellites; and the National Imagery and Mapping Agency, which interprets satellite imagery and makes military maps.

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